1. Field of the Invention
The present invention relates to an image data processing apparatus and an image data processing process which are to be applied, for example, to a digital color copying machine capable of adjusting the quality of an image.
2. Description of Related Art
Digital color copying machines typically include a scanner section for reading an original image and outputting image data, an image processing section for processing the image data outputted from the scanner section, and an image formation section for forming an image on the basis of the image data processed by the image processing section. The scanner section, for example, includes a color CCD (charge coupled device) for outputting signals on the basis of the additive process of three primary colors including red (R), green (G) and blue (B), and a color conversion section for converting the signals outputted from the color CCD into image data on the basis of the subtractive process of three primary colors including cyan (C), magenta (M) and yellow (Y). The image processing section appropriately processes the signals outputted from the scanner section, and generates black (BK) image data. The image formation section includes a laser scanning section for outputting a laser beam on the basis of the color image data for the aforesaid four colors, and is adapted to form a color image with the use of four color toners, i.e., cyan, magenta, yellow and black color toners, on the basis of an electrophotographic process.
In the digital color copying machines, the density gradation levels of respective pixels constituting a document original image should be precisely reproduced to reproduce the image with a high fidelity. If image data outputted from the scanner section are directly applied to the laser scanning section, density gradation levels cannot precisely be reproduced due to the characteristics of the toners and the photoreceptor. Therefore, some digital color copying machines have a function for automatically adjusting the density gradation levels of an output image by correcting the image data in order to reproduce the original image with a high fidelity.
More specifically, an output gradation reference curve prepared in consideration of the toner characteristics, the photoreceptor characteristics and the like is stored in the form of a table in a memory. The output image data corresponding to the inputted image data are generated on the basis of the table.
Digital color copying machines are used not only for reproducing an original image with a high fidelity but also for processing an original image with some intention. For example, an image with clear characters can be formed by processing image data so that black characters in an image are enhanced. To facilitate such an image processing, some digital color copying machines offer, for example, four image quality modes including a character mode, a map mode, a photography mode, a character/photography mode, and are adapted to automatically change the quality of an image to be formed when one of these modes is selected.
Another exemplary image processing is to render a full-color original image into a monochrome image. Some digital color copying machines offer a full-color mode in which a full-color image is formed with the use of four color toners, and a monochrome mode in which a monochrome image is formed. The monochrome mode includes seven modes, i.e., modes adapted for forming a cyan, magenta, yellow or black monochrome image with the use of a single color toner, and modes adapted for forming a red, green or blue monochrome image with the use of two color toners. Such digital color copying machines are adapted to automatically change the color of an image to be formed when one of the eight color modes is selected.
In a digital color copying machine having the aforesaid image quality modes and color modes, its image processing section should process image data inputted from the scanner section in different manners depending on the selected mode so that an image having a different image quality and color is formed depending on the selected mode. In the full-color mode in which the four color toners are used, four images for the four colors, i.e., C, M, Y and BK, are sequentially formed, and transferred one on another on a transfer sheet. At this time, since toner particles cannot easily adhere onto the transfer sheet bearing one color toner image transferred thereon, the amount of each color toner adhering onto the transfer sheet is smaller than that in the monochrome mode using a single color toner. Therefore, where a single color toner is used, for example, image data to be outputted should have lower density gradation levels than the case where the four color toners are used.
Since the four image quality modes may be each combined with the eight color modes, 32 (=4.times.8) tables should be stored in the memory to cover all the modes for automatic gradation adjustment of an output image.
In addition, the mechanical characteristics vary from machine to machine and, hence, the tables for the output image gradation adjustment are prepared for each machine and installed therein.
Further, since the mechanical characteristics change with the lapse of time, the tables should be corrected by a technical service person.
However, the installation or correction of the 32 tables for each machine is a time-consuming task, which prevents the improvement in the productivity and the efficiency of the correction operation to be performed by the technical service person. Further, a large capacity memory is required for the storage of a large number of tables, preventing the cost reduction.